Travelling walve amplifier



June 18, B. KAZAN TRAVELLING WAVE AMPLIFIER Filed July 5, 1951 F|G.l

JNVENTOR. BENJAMIN KAZAN y: y

FIG 2 WMM ilnited States Patent TRAVELLING WAVE AMPLIFIER Benjamin Kazan, Long Branch, N. J., assignor to the United States of America as represented by the Secretary of the Army Application July 3, 1951, Serial No. 235,087

v 13 Claims. (Cl. S15-3.5) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This invention relates to high frequency amplifiers of the travelling wave type and more particularly to travelling wave amplifiers with homogeneous wave propagating means.

Travelling wave tubes heretofore known include a conductor either in the form of a helix or a series of interconnected bafiies past which an electromagnetic wave can propagate. The phase velocity of the electromagnetic wave as measured along the axis of the conducting structure is small compared to the velocity of the Wave along the surface of the conductor. An electron beam source is arranged to direct a stream of electrons adjacent the conductor, and where the conductor is in the form of a helix, the beam is usually directed axially through the helix. in operation, the axial phase velocity of the electromagnetic wave is so related to the axial path of the electrons that the electrons travel at a slightly higher velocity than the phase velocity of the wave. This dilerence in travel velocity results in part of the kinetic energy of the electrons being converted into radio frequency energy and the electromagnetic wave propagated in the direction of the electron ow is thereby amplified. Such travelling wave tubes are particularly suited for employment as an amplifier of microwave frequencies. However, because of the inherent discontinuities in the conductor path of such amplifiers, and the diiiicnlty in causing the electrom beam to travel very close to the conductor (a fraction of a wavelength), amplification of radio frequency energy of millimeter wavelength is very diilicult to achieve.

It is therefore an object of the present invention to provide a travelling wave type amplifier capable of amplifying radio frequency energy of millimeter wavelength.

Itis another object of the present invention to provide a travelling wave type amplifier employing homogeneous wave transmission means.

lt is still another object of the present invention to provide a travelling wave type amplier wherein a homogeneous wave slowing structure of dielectric material is employed as the wave transmission means.

ln accordance with the invention, there is provided a travelling wave tube amplifier comprising an elongated homogeneous wave transmission means of dielectric material and an electron source for producing parallel streams of electrons of uniform intensity and velocity which are projected toward the surface of said homogeneous transmission means at a predetermined angle. A screen wire mesh or grid, is positioned across the electron path in close proximity to the wave transmission means to provide a collector electrode.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing in which:

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Figure 1 is a view, partly in section, of a travelling wave amplifier in accordance with my invention;

Figure 2 is a fragmentary view in perspective showing the wire mesh and transmission means employed in the amplifier of Figure 1;

Figure 3 is a fragmentary view in perspective showing a grid type structure as the collector means;

Figure 4 is a fragmentary view in perspective illustrating collector plates employed with the transmission means when the secondary emission ratio is greater than unity;

and

Figure 5 is a fragmentary view of the elements of Figure l illustrating a preferred mode of operation.

Referring now to Figure l of the drawing there is shown a travelling wave amplifier including an evacuated envelope 2 enclosing an electron gun 4, a homogeneous transmission line 6 and a grid, or screen wire mesh 8.

Homogeneous transmission line 6 may comprise an elongated rod or slab made of suitable dielectric material having a dielectric constant greater than unity. The purpose of employing material with a dielectric constant greater than unity is to obtain a relatively low velocity of wave propogation. In Figure l, transmission line 6 is shown as a slab of dielectric material positioned in a plane parallel to the longitudinal axis of envelope 2. It is to be understood, of course, that transmission line 6 may be disposed within envelope 2 in any other suitable position and may be of any desired shape in cross section, though here a rectangular shape is assumed as indicated in Figure 2.

Electron gun 4 may comprise a conventional electron emitting source and suitable focussing and accelerating electrodes, not shown, for projecting parallel streams of electrons 10 of desired current strength and velocity. Since such electron gun structures are generally Well known, complete details of the gun and its power supply circuits are not shown in the drawings in order to avoid unnecessary complication. As shown, electron gun 4 is positioned with respect to transmission line 6 so that the parallel electron streams 10 are projected toward one surface of the dielectric transmission line, hereinafter referred to as the exposed surface, at a suitable angle. Thus, each of the parallel electron streams may be considered to have a component of velocity parallel to the exposed surface of the dielectric transmission line and a component of velocity normal to the exposed surface. If desired, a magnetic field,`not shown, may be provided in the usual manner as an additional control for maintaining the streams of electrons from the gun in parallel lines.

Spaced above the exposed surface of transmission line 6 in close proximity thereto, there is provided a screen wire mesh 8, the details of which are clearly illustrated in Figure 2. If desired, screen wire mesh 8 may be replaced by a simple grid structure 7 as shown in Figure 3 or any other suitable foraminated structure. As shown, grid or screen wire mesh 8 is coextensive with and parallel to said exposed surface and is maintained at a positive direct-currentpotential with respect to the electron emitting source of electron gun 4 by means of battery 12 or any other Vsuitable source well known in the art. As will be hereinafter explained, the applied direct-current potential may be of a magnitude between the rst and second cross-over potentials of the dielectric which comprises the transmission line to provide a secondary emission ratio greater than unity, or preferably, of a magnitude greater than the second cross-over potential to provide a secondary emission ratio less than unity.

Evacuated waveguide portions 14 and 16 are provided to conduct the electromagnetic input and output energies of the amplifier through coupling Windows V18 and 20 respectively. Guide portions 14 Vand 16 may be parte 9i incoming and outgoing wave guides, the input guide, of which 14 would -be a part, extending indenitely to the left and the output guide, of which 16 would be a part, extending indefinitely to :the right. -lt listo be understood f1co1lrse,tthat any other suitable method of conducting energy to and from the coupling guide portions maybe employed. The ends of transmission line 6 are tapered in two directions to provide ,proper impedance matching of the dielectric and the waveguide coupling portions 14 and 1,6.

In operation, the input wave .of yelectromagnetic energy is propagated along transmissionline 6 at a velocity determined by the dielectric constant of the dielectric. it is well known that part of the :total lield associated with propagation valong a dielectric transmission line may rel side outside the line itself, if the .cross-section is suiiiciently small compared ,to la wavelength. It isalso wel] known that when a solid ,dielectric is bombarded by a stream of electrons, the secondary emission .ratio lfrom the dielectric is a function of the Vprimary electron energy expressed in electron volts. ABetween approximately 100 volts and V100() volts, the secondary emission ratio vis greater than unity and lbelow 100 volts and above 100() volts the secondary emission -ratio is ,less than unity.k The lower and higher values of electron energycorresponding to a secondary emission ratio of unity are designated as first and second cross-over potentials, respectively, of the dielectric.

Parallel streams of electrons are projected angularly from electron gun .4 through grid lor screen wire mesh 8 to impinge on the exposed surface'of transmission line 6. If the directe-current potential applied between wire mesh 8 and the electron emitting source of gun 4 is assumed to `be Ibetween the irst and second cross-over potentials of the dielectric, -i. e., the secondary emission ratio is greater than-unity, the exposed surface of the dielectric will assume a potential approximately equal to that of the wire mesh. Thus, effectively, the surface of dielectric transmission line 6 is substantially at the same potential as the grid or wire mesh with vrespect ,to the electron emitting source. Those of the secondary electrons produced Vat the surface of the dielectric, which escape from the surface, are collected by wire mesh 8 which is at positive potential with respect to the electron emitting source. With a secondary emission greater than unity, conventional collector plates may be utilized in place of the grid or wiremesh 8. A collector plate 9 may beproximately positioned on each side. ofthe .dielectric substantially coextensive therewithkas shown in Figure 4.

If vit Vis to berassumed that the applied direct-current potential is of such magnitude that the component of velocity of each electron stream parallel to .the exposed surface of the transmission line is approximately equal to the phase velocity of the electromagnetic wave propagated along the dielectric transmission line, interaction will occur at fany time between those electrons just arriving at and in -the neighborhood of the dielectric, and the electromagnetic wave ,outside `the dielectric.V Until the electrons impingetdirectly on the exposed surface ofthe dielectric transmission line, there is a very high degree of coupling between the propagated electromagnetic wave and the electrons. As soon .as theelectrons strikethe dielectric, they no longerttakepart in the R.F. interac tion. Howevensince the R.F. .energy from any portion of the dielectric propagates to the next portion, eachelement of the electron -beam during its'time of interaction serves to add to the growing `R.F. wave before it impinges in the dielectric transmission line.

The invention may also operate asa travelling wave amplifier when the magnitude of the appliedV direct-current potential is, for example, greater than the second cross-over potential of `the dielectric so that as the electrons pass .the screen -mesh they'have a velocity corresponding to a secondary emission ratioV from thedielectric of lessv than unity.V Under these conditions, the dielectricwillautomatically charge negatively to apotential slightly less than .the grounded wire mesh. Electrons entering the space between the wire mesh and the exposed surface of the dielectric transmission line will thus have their vertical velocity component reduced to zero when they are very close to .the exposed surface and then be reflected toward the wire mesh as shown in Figure 5. However, since the decelerating field between the wire mesh and the exposed surface of the dielectric transmission line is assumed to exist only in the vertical direction, the electrons will ,be reiiected .towards the Wire mesh without any change in their horizontal velocity.

As an example, let it be assumed that the potential of the electron source is Ysuch with respect to mesh 8 that the electrons have a componentof velocity perpendicular to the surface of vthemeshA of lOQOvolts. Dielectric transmission line' 6 `Vwill then assume a potential of GOOG-ka) volts with respect to the wire mesh where a is a small positive quant-ity dueV to the spread in emission velocities by the emitting sources. The electron beams will thusybe reflected at points very close to the surface of the dielectric Vtransmission line. inasmuch as the 'horizontal component .of velocity is not affected by this action and theelectron beams pass very close to the dielectric transmission line, a travelling wave type of interaction will occur and radio frequency signals fed into one end of the wave slowing device will emerge ampliiied at theother end,

While there have been described what at present are considered to beY the preferred embodiments of the invention, it will 'be understood by those skilled in the art that various changes and modifications may be made herein without departing from Vthe invention, and it is, therefore, aimed in the appended claims to cover all such modifications and `changes as fall within the spirit and scopeof the invention.

What is claimed is: ,Y y

l. A travelling wave amplifier comprising an elongated homogeneous transmission means of dielectric material along which electromagnetic wave energy maybe propagated with a propagation velocity dependent upon physical characteristics of said transmission means, means for directing a stream of electrons toward said transmission means at a predetermined angle, said stream of electrons impinging onat least oneV surface Vof `said transmission means, a wire meshproximately spaced from said transmission means and coextensive therewith, said wire mesh being disposed between said electron directing means and the impinged surface of said transmission means, a direct current voltage source connected 'between said wire mesh and said electron directing means for projecting said electron stream at a predetermined velocity such that the velocity component of said electron stream parallel to the surface of said transmission means issubstantially the same as said propagation velocity.

2. A travelling wave amplifier in accordance with claim l wherein said homogeneous transmission means has a dielectric constant greater than unity.

3. A travelling wave ampliferin accordance withL claim 2 wherein the secondary emission ratio of the dielectric is greater than unity.

4. A travelling wave aniplier comprising a homogeneous dielectric transmission means along which electromagnetic wave energy may be propagated with a propagation velocity dependent upon thevdielectric constant of said transmission means, means for angularly projecting a plurality of parallel electron streams toward said transmission means at a predetermined velocity such that the velocity component of said electron streams parallel to the surface of said transmissionfprneans is substantially the same as said propagationvelocity, and grid means proximately spaced from said transmission means and coextensive therewith, said grid means Vbeing disposed between said electron .projecting means and said transmission means. Y

5. A travelling wave amplifier inaccordance with claim 4, wherein the secondary emission ratio of the dielectric is less than unity.

6. A travelling wave amplier comprising means for projecting parallel streams of electrons, each of said streams having a predetermined horizontal velocity component and a vertical velocity component, a source of electromagnetic energy, homogeneous transmission means along which said electromagnetic energy is propagated with a propagation velocity substantially the same as said horizontal velocity, said transmission means being disposed relative to said electron projecting means whereby said parallel electron streams impinge angularly on said transmission means for substantially the entire length thereof, and a wire mesh disposed between said transmission means and said electron projecting means and coplanar with said transmission means, said wire mesh being proximately spaced from said transmission means.

7. A travelling wave amplifier in accordance with claim 1 wherein said wire mesh is parallel to the impinged surface of said transmission means.

8. A travelling wave ampliiier comprising an elongated homogeneous transmission line of dielectric material, a source of parallel streams of electrons, means for directing said parallel stream of electrons toward said transmission line at a predetermined angle whereby said stream of electrons impinge on at least one surface thereof, and foraminated means proximately spaced from the impinged surface of said transmission line and disposed between said impinged surface and said electron directing means.

9. A travelling wave amplier in accordance with claim 8 wherein said foraminated means is parallel to said impinged surface and coextensive therewith.

10. A travelling wave amplifier comprising means for projecting parallel streams of electrons, each of said streams having a horizontal velocity component and a vertical velocity component, homogeneous dielectric transmission means adapted to propagate electromagnetic wave energy with a propagation velocity substantially equal to said horizontal velocity, said transmission means being disposed relative to said electron projecting means whereby said parallel electron streams angularly impinge on at least one surface of said transmission means for substantially the entire length thereof, and foraminated means proximately spaced from said transmission means, said foraminated means being parallel to the impinged surface of said transmission means and disposed between said electron projecting means and said transmission means.

11. A travelling wave amplifier comprising an elongated homogeneous transmission means of dielectric material adapted to propagate electromagnetic wave energy with a predetermined propagation velocity, means for directing a plurality of parallel electron streams toward said transmission means at a predetermined angle whereby said streams impinge on at least one surface of said transmission means, a foraminated structure parallel to and proximately spaced from the impinged surface, said foraminated structure being disposed between said electron directing means and said impinged surface, a direct-current voltage source connected between said foraminated structure and said electron directing means for projecting said electron streams at a predetermined velocity such that the velocity component of said electron streams parallel to said impinged surface is substantially equal to said propagation velocity.

l2. A travelling wave amplilier comprising an evacuated cylindrical envelope, an elongated homogeneous transmission line of dielectric material positioned in a plane parallel to the longitudinal axis of said envelope and adapted to propagate electromagnetic wave energy with a predetermined propagation velocity, means for directing a plurality of parallel electron streams toward said transmission line at a predetermined angle to impinge on at least one surface of said transmission line whereby each of said streams has a horizontal velocity component parallel to said impinged surface and a vertical velocity component normal to said impinged surface, a foraminated structure parallel to and proximately spaced from the impinged surface, and a direct-current voltage source connected between said foraminated structure and said electron directing means for projecting said electron streams at a predetermined velocity such that said horizontal velocity component is substantially equal to said propagation velocity.

13. A travelling wave amplifier comprising an evacuated cylindrical envelope, an elongated, homogeneous transmission line of dielectric material and rectangular in crosssection positioned within said envelope and adapted to propagate electromagnetic wave energy with a prescribed propogation Velocity, said dielectric material having first and second crossover potentials, means for directing a plurality of parallel electron streams toward only one surface of the transmission line at a predetermined angle whereby each of said streams has a horizontal Velocity component parallel only to said one surface and a vertical component normal to said one surface, a foraminated structure parallel to and proximately spaced from said one surface of said transmission line, and a source of directcurrent potential of a magnitude greater than said second crossover potential connected between said foraminated structure and said electron directing means for projecting said electron streams at a prescribed velocity such that said horizontal velocity component is substantially equal to said propagation velocity.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,538 Potter July 5, 1938 2,304,540 Cassen Dec. 8, 1942 2,367,295 Llewellyn Ian. 16, 1945 2,559,581 Bailey July 10, 1951 2,611,101 Wallauschek Sept. 16, 1952 2,636,148 Gorham Apr. 21, 1953 2,661,441 Mueller Dec. 1, 1953 FOREIGN PATENTS 508,354 Great Britain June 29, 1939 

